To tackle the problems related to inter-symbol interference (ISI) and the Doppler effect, there is a well-known way of adding a guard interval, in which no information is transmitted.
A major drawback of this technique is that it has reduced spectral efficiency, since no payload information is transmitted during this interval. A technique is therefore sought to reduce the ISI affecting the multicarrier signals without introducing any guard interval.
A technique has been proposed in the French patent document FR 2 814 302 filed on behalf of the present Applicant for the construction of a multicarrier signal, for example of the OFDM/OQAM type, that does not necessitate the introduction of a guard interval and makes it possible to limit the interference affecting the pilots. It may be recalled especially that OFDM/OQAM (offset QAM) modulation is an alternative to the classic OFDM modulation and, unlike OFDM modulation, does not require any guard interval.
In this technique, the multicarrier signal, comprising pilots and informative data elements is built by fixing the value of at least one of the informative data elements on a ring comprising the carriers that are immediately adjacent to a pilot so as to reduce the interference affecting this pilot at reception.
Indeed, the inventors have noted that the intrinsic interference affecting the reference carriers of the time-frequency plane, called pilots, depends chiefly on the carriers that are immediately adjacent to each of the pilots.
Thus, as described in detail in Section 2 below, which is an integral part of the present patent application, the interference Cm0, n01 due to the eight carriers forming a ring surrounding a pilot, called a first ring, in the French patent application FR 2 814 302 can be written as follows:
      C                  m        0            ,              n        0              1    =            ∑                        (                      m            ,            n                    )                ∈                  Ω                                    m              0                        ,                          n              0                                1                      ⁢                  a                  m          ,          n                    ⁢                        ∫                          ⁢                                            g                              m                ,                n                                      ⁡                          (              t              )                                ⁢                                    g                                                m                  0                                ,                                  n                  0                                            *                        ⁡                          (              t              )                                ⁢                      ⅆ            t                              with:                am, n being the real symbols sent by the mth sub-carrier of the nth symbol time;        Ωm0, n01 being indices of the carriers immediately adjacent to a pilot;        gm, n(t) the time-shifted and frequency-shifted versions of the prototype function g(t).        
The inventors have therefore sought to cancel the intrinsic interference due at least to the first ring on certain reference carriers of the time-frequency plane, especially for an OFDM/OQAM type multicarrier signal.
According to the technique presented in the document, a relationship between the carriers of the first ring surrounding a pilot is introduced to eliminate the intrinsic interference locally (and almost totally) and enable an accurate estimation of the propagation channel.
In other words, this document proposes a technique for the reduction of interference phenomena implementing a multicarrier modulation with distributed pilots, in which one or more constraints are imposed on the value of one or more informative data elements that are to be transmitted, so as to reduce the ISI detrimental to the implementation of an accurate channel estimation.
Thus, the interference affecting a given pilot due to the carriers belonging to the first ring surrounding this pilot, i.e. due to the carriers that are immediately adjacent to the pilot considered, in the time space on the one hand and in the frequency space on the other hand, is cancelled.
However, one major drawback of this technique is that it is not suitable when the number of pilots increases. Now, in order to improve the signal-to-noise ratio, it is advantageous to increase the number of pilots sent out per antenna, scattered in the multicarrier signal.
Unfortunately, in this prior art technique of generation of a multicarrier signal, the rings surrounding each of the pilots must be disjoined, and this makes it necessary for the pilots to be separated by at least two carriers in time and/or in frequency.
More specifically, as illustrated with reference to FIG. 7, increasing the number of pilots gives rise to an increase in the number of rings around these pilots and the number of fixed values.
Thus, the use of a ring around each of the pilots soon induces a problem of space in the time-frequency plane. In other words, increasing the number of pilots give rise to a more important use of the time-frequency resource.
Similarly, this prior art technique is not suited to transmission in a MIMO or MISO type system in which several pilots associated with different antennas are positioned at distinct locations in the time-frequency plane in order to estimate the propagation channels between the transmission and reception antennas.
Indeed, in the context of multi-antenna transmission, pilots are associated with each of the transmit antennas thus increasing the number of pilots.
In order to enable accurate estimation of the propagation channel, it is therefore indispensable that, at the time-frequency location of a pilot of an antenna, there is no interference or jamming induced by the neighboring antenna.
Unfortunately, the prior art technique does not deal with this case and is consequently ill-suited to transmission with several antennas.